1. Field of the Invention
The present invention relates to a process for the injection of a system of one or more non-precatalyzed, polymerizable, monocomponent or hybrid resins at high pressure and at high flow.
2. Description of the Prior Art
In the field of injection molding, many different processes have been developed in an attempt to economically produce molded articles. It is well known in Resin Transfer Molding (RTM) technology that a flow consisting of a thermosetting monomer and catalyst (monocomponent system) can be premixed in a proper mixing head and pumped at low pressures (2-5 atmospheres) into a properly preheated mold where the polymerization reaction will occur.
The use of a low molding pressure permits the use of less expensive molds which are made of relatively weak materials. However, the low molding pressure makes the process quite slow, particularly when the molded .articles are large. With this type of process, the maximum delivery of the metering pumps are not very high and typically do not exceed 100-150 cc/second.
Furthermore, after the conclusion of the mixing/injection (shot) step it is necessary to use a solvent to clean the mixer. Obviously this slows the manufacturing cycle and creates the need for disposal of the used solvents.
To overcome these drawbacks, monocomponent resin/catalyst systems have been prepared which are endowed with rapid polymerization kinetics and which are characterized by total cross-linking times of less than 5 minutes. These systems, however, are also significantly slowed by the low pressure injection step, which results in a significantly slower process than that potentially permitted by the above-mentioned kinetic characteristics.
A further method of reducing production cycle time in injection molding processes uses RIM (Reaction Injection Molding) machines. These machines mix the resin and catalyst flows by injecting them separately at high pressures into a mixing chamber. The RIM technology is based on the impingement of two opposed flows injected by means of suitable nozzles in a mixing head equipped with a self-cleaning piston system. The RIM system eliminates the need to clean the chamber after each production cycle. Additionally, the self-cleaning piston is capable of expelling from the chamber the residual material after the shot.
When the RIM technology is applied to monocomponent resins the latter are divided into two flows. One is composed of basic resin and accelerator while the other is composed of basic resin and catalyst. These two flows are mixed in the RIM mixing chamber in opposing flows.
This method, although it permits acceleration of the monocomponent resin molding cycle, results in instability because one flow of resin is already catalyzed. This instability, creates difficulties during the storage of the flow prior to mixing. Additionally, it can give rise to undesired polymerizations resulting from localized overheating phenomena which can occur during the process at different points in the production plant.
One possible solution to the problems resulting from pre-mixing the resin and catalyst is to keep them separate until the resin is actually present in the mold. In other words, the resin and the catalyst are mixed in the mold. Two separate flows, resin and catalyst, are injected into a mixing chamber where the polymerization reaction can take place. After the reaction is complete, the self-cleaning piston ejects the molded article from the chamber and the next cycle can begin.
The major challenge in such a system is to ensure that the flows mix properly so that proper catalyst dosage is given to the resin flow in all regions of the mixing chamber. This step is critical, since failure results in less than optimal polymerization conditions in the mixing chamber, which in turn creates poor mechanical properties in the final product.
One method of solving the mixing problem is exemplified by Japanese Patent Document JP 60-193624 in which two separate resin flows are injected into the mixing chamber. The catalyst is injected through an injection nozzle which is positioned so that the catalyst is injected directly at the resin injection nozzle. This puts the resin and catalyst flows in "counter-current" with each other.
In practice the mixing of the flows obtained by placing them in counter-current with each other is not satisfactory because it results in catalyst segregation and poor properties in the final product. Furthermore, the opposed positioning of the resin and catalyst injection nozzles as exemplified by JP 60-193624 creates a tendency for unpolymerized resin to become lodged in the catalyst injection nozzle, where it subsequently polymerizes and clogs the nozzle.
Other devices in which the resin and catalyst flows are not directly opposed, such as German Patent Document DE 2,314,459, do not overcome the mixing problems which lead to less than ideal final product quality.